Purple Aurora Majesties

The summer Milky Way rises along the eastern horizon last night around 11:30 p.m. The slight green glow at bottom is airglow, caused by air molecules emitting light after being excited to higher energy levels by sunlight during the day. Photo: Bob King

Last night a stranger approached me in the dark. I was out in the country in a pullout along a gravel road just setting up my telescope. You never know what to expect when someone drives up to you and steps out of their car. I make the assumption their motivation is curiosity and not harm. So far, that’s been true.

We approached each other, gave names, shook hands. Both of us relaxed. Over the next half hour Tony and I shared looks at Saturn, Venus, a globular cluster, two galaxies, a meteor and one of the brightest Iridium satellite flares I’ve ever seen in my life.  At magnitude -8 or nearly as bright as a half moon, the flare demonstrated how bright a nearby supernova might look.

Unusual purple auroral rays briefly stripe the northern sky last night about 10:30 p.m. The more common green aurora is below. Photo: Bob King

We asked questions about each others interests and professions and remarked about the lone wolf howling a mile away. Then he decided it was time to go and offered an apology for disturbing me. None was needed. There’s always joy in sharing the sky. By connecting to something larger than ourselves, we find we’re more alike than different.

Later I was happy to see the smouldery glow of aurora beyond the trees to the north. Nothing bright or spectacular, just a few patches and rays. But I soon learned this was no ordinary aurora. Most auroras are pale green to the eye and shine a vivid green in time exposure photos. These rays appeared colorless, but as soon as I pressed the button to check the exposure, my jaw dropped. Purple!

A striking purple ray from last night caused by excited nitrogen molecules high in Earth's atmosphere. Photo: Bob King

Auroral colors are generated when those frisky electrons from the sun follow Earth’s magnetic field lines straight into the polar atmosphere. There they strike the the atoms and molecules of oxygen and nitrogen and pump them up to an excited state.

In the atomic world, excitement means an electron that orbits close to the nucleus of the atom gets bumped up to a higher energy level. These higher energy states don’t last for long. An excited atom typically crashes into neighboring atoms and loses its energy, but in the near vacuum of the upper atmosphere, atoms don’t meet up very often. Instead, the excited electron returns to its “rest” state, releasing a particle of light called a photon in the process. Photons are packets of energy that make up a beam of light.

Oxygen in the lower atmosphere, from about 50 miles to 120 miles high, emits photons of green light when pounded by solar electrons. Since the element is plentiful and our eyes particularly sensitive to green light, the color dominates most auroras. Picture billions of oxygen atoms beaming bits of green light and you’ve got a typical northern lights display.

Oxygen atoms from 100 miles to as high as 375 miles, where the air is even more rarefied, create the less often seen red auroras. Picture energy levels in an atom as floors in a hotel. When hit by a solar particle, an electron in an oxygen atom “takes the elevator” to a higher floor. Green light is emitted when the electron rides the elevator down from the third to second floor; red light when electrons ride from second floor to first (ground level).

Oxygen atoms are primarily responsible for the greens and reds of the aurora; nitrogen gives us red-bottomed arcs and tall purple rays. Credit: WebExhibits

Nitrogen atoms and molecules give off several colors when stimulated by incoming material from the sun, most of which we can’t see with the naked eye. A few nitrogen atoms emit a faint green that gets lost in oxygen’s brighter colors, but in intense auroras, you’ll sometimes see an intense deep red-purple fringe along the bottom of rays and arcs. This captivating color is caused by nitrogen.

Last night’s aurora was even more unusual because it not only came from very high up but also involved sunlight. Purple light is cast off in a two-step process. First, nitrogen molecules got whacked by electrons, rise to an excited state and then dump their light as purple photons. Second, the excited nitrogen absorbes energy from sunlight shining onto the very top of the atmosphere, exciting the molecules to pump out even more purple. All this happens some 600 miles high!

Too bad our eyes aren’t very sensitive to the blue-purple end of the spectrum, otherwise nitrogen-producing auroras would be much more spectacular. As it was, I saw only a faint hint of blue in the brightest ray last night.

Let’s think about the aurora process again. Electrons stream from the sun and their energy of movement (kinetic energy) is transformed into light via the elevator process described earlier. Second, once the nitrogen molecules get into their excited state, sunlight itself was enough to do the job. That boggles the mind. Simple things like atoms and molecules are a type of machine able to transform one form of energy into another.

Speaking of rarefied, the air is nearly a vacuum at the height the aurora takes place with a pressure one-millionth what it is at sea level. Were it not for the fact that we look through hundreds of miles of light-emitted oxygens and nitrogens, these tiny emissions be far too faint to see.

Just in time, the NOAA space weather forecast predicts possible auroral activity over the next three days.

6 Responses

  1. Dan Gendreau

    I live about 13 miles north of Park Rapids.

    In my position while watching the television, I am facing south with an array of three windows to my right which give me a pretty good view of the SSW, SW and West sky. At a few minutes before ten this evening my eyes were drawing away from the screen to an exceptionally bright meteor which appeared initially as a bright white ball trailing a illuminated tail. The ball was at least thee to four times the size of the brightest appearances of Venus and at least 10 times a bright.
    It was bright enough when observed from a lit room that it reminded me of watching fireworks. It appeared in the upper most southern corner of my field of view and moved across the sky, in a very straight path, in a descending westerly or north westerly direction. When it reach a point where it was due SW and in about the middle of my field of view it broke up into many smaller pieces several of which were still as bright as an evening star (planet). Shortly they extinguished. It was not moving very fast as meteors go with the entire show lasting approximately a second and a half to traverse approximately 45 degrees of my field of view.

    I wonder if anyone else saw it.

  2. Miki T-B

    My husband and I and two friends were on our way home to Cooperstown from Larimore last night and saw the same thing. We’d never seen anything like it. It was, like, WAY COOL! Two of my co-workers also saw it last night.

  3. Tone Coughlin

    Hi Bob, thanks for showing me the planets and stars. I didn’t know there was an aurora out too. Too bad I missed it!

    Cool photos!

    1. astrobob

      Hi Tone,
      So you found the blog! Don’t worry too much about the aurora – it was faint and didn’t last long. Much better in the camera which is more sensitive to those kinds of things. It was a pleasure to share the sky – glad you stopped by.

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